1. Field of the Invention
The present invention relates to a tennis racket frame and more particularly, to a tennis racket having a favorable repulsion performance and ball control performance and giving a soft ball-hitting feeling to a player by improving the cross sectional configuration of a string-installing portion of the frame thereof.
2. Description of the Related Art
Research has been made to improve the performance of the tennis racket and as a result, tennis rackets in various configurations have been proposed. The configurations of the tennis rackets developed recently are classified into the following three types:
1. Mid (standard) size racket; PA1 2. Rackets having a large ball-hitting surface, namely, the so-called large racket; and PA1 3. Rackets having a large vertical sectional area of a string-installing section of the racket frame, namely the so-called thick racket. PA1 (1) Spring shown by (A) of FIG. 15 generated by the deformation of strings 1[.]; PA1 (2) Spring shown by (B) of FIG. 15 generated by the deformation of a supporting portion (string-installing portion) 2 for supporting the strings 1; PA1 (3) Spring shown by (C) of FIG. 15 generated by the in-plane deformation of a racket frame 3; and PA1 (4) Spring shown by (D) of FIG. 15 generated by the out-of-plane deformation of the racket frame 3.
A tennis racket is required to have a favorable repulsion performance and ball control performance and provide a soft ball-hitting feeling to a player. The main factor for determining these performances is the spring characteristics.
The spring characteristics are classified into the following four types as shown in FIGS. 15A, 15B, and 15C in consideration of the construction of the racket.
It is considered that the above four springs are connected in series with each other and hence a most deformable spring determines the characteristic of the racket.
Observing the deformation of the mid (standard) size racket which occurs when a tennis ball collides with the ball-hitting surface thereof, the racket frame 3 is deformed like a spoon as shown in FIG. 15C. The spring (D) generated by the out-of-plane deformation of the racket frame 3 is the main factor for determining the characteristic of the racket.
Because the ball-hitting area of the mid size racket is smaller than that of the large racket, the in-plane rigidity of the former is higher than that of the latter and thus, the stability of ball-hitting surface of the former is more favorable than that of the latter and hence, ball control performance of the former is higher than that of the latter.
Because the thickness (h) of the string-installing portion 2 is smaller than that of the thick racket, the out-of-plane rigidity of the former is low and thus, the mid size racket is flexible and gives a soft feeling to the player in hitting a tennis ball.
As described above, the mid size racket has a favorable ball control performance and gives a soft feeling to the player in hitting the tennis ball, but the spring main factor for determining the characteristic thereof is generated due to the spring (D) caused by the out-of-plane deformation of the racket frame 3. The spring (D) does not greatly contribute to the improvement of the repulsion performance of the racket. The large racket and the thick racket have been developed to improve the repulsion performance of the racket.
In the large racket having a large ball-hitting area, the spring (A) generated by the deformation of the strings 1 is the main factor for determining the characteristic thereof and thus the large racket has a favorable repulsion performance.
In the thick racket in which the thickness (h) of the string-installing portion 2 is great, the main factors for determining the characteristic thereof are the spring (A) generated by the deformation of the strings 1 and the spring (B) generated by the deformation of string-installing portion 2. In particular, curved peripheral surfaces of the string-installing portion 2 are deformed and thus a strong spring generated due to the return of the deformation of the curved peripheral surfaces displays a higher repulsion performance than the large racket.
The string-installing portion 2 of the conventional tennis racket frame has an approximately rectangular, sectional configuration as shown in FIG. 16A; an approximately, octagonal sectional configuration as shown in FIG. 16B; or an approximately elliptical, sectional configuration as shown in FIG. 16C. The frame has, on the center of the outer side of the string-installing portion 2, a concave portion 2a into which a grommet used to install a string thereon is inserted; and has gut holes 2b and 2c on the center of the bottom surface of the concave portion 2a and the inner side of the string-installing portion 2 opposed to the center of the bottom surface of the concave portion 2a, respectively.
The thickness (h) of the mid size racket and that of the thick racket are approximately 20 mm and 30 mm at the largest portion thereof, respectively.
The ball-hitting area of the mid size racket and that of the large racket are approximately 93 to 95 square inches and 105 to 108 square inches, respectively.
As described above, the large racket and the thick racket have a higher repulsion performance than the mid size racket, respectively, whereas they have a lower ball control performance than the mid size racket and give a less soft ball-hitting feeling to the player than the mid size racket for the reason which is described below.
That is, because the large racket has a larger ball-hitting area than the mid size racket, the in-plane rigidity of the ball-hitting surface of the large racket is lower than that of the mid size racket and thus the deformation amount of the in-plane deformation of the former is greater than that of the latter. Thus, the stability degree of the ball-hitting surface of the large racket is inferior and thus the ball control performance thereof is unfavorable.
In the thick racket, the deformation of the spring (B) generated by the deformation of the string-installing portion 2 is restored in a shorter time period than the other springs (A), (C) and (D). Thus, the period of time in which the thick racket and the ball are in contact with each other is short and thus the ball control performance thereof is unfavorable.
In addition, because the thickness (h) of the thick racket is great, the thick racket does not generate the out-of-plane deformation, thus giving a hard ball-hitting feeling to the player when the player hits the ball with the thick racket. Impacts generated in ball hitting are transmitted to the arm of the player. Hence, when the player continues to use the thick racket for a long time, the player may develop a tennis elbow on the arm or the elbow.